1. Field of the Invention
The present invention generally relates to image rejection in a dual conversion receiver, and more specifically to improving image rejection using a feed-forward network that cancels the image frequency at the output of a channel selection filter.
2. Background Art
Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHZ, with 6 MHZ channel spacings in the United States and 8 MHz channel spacing in Europe. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.
Regardless of the transmission scheme, a tuner is utilized to select and down-convert a desired channel from the TV signal to an intermediate frequency (IF) signal or a baseband signal, which is suitable for processing and display on a TV or computer screen. The tuner should provide sufficient image rejection and channel selection during down-conversion as is necessary for the specific application. The National Television Standards Committee (NTSC) sets standards for television signal transmission, reception, and display. To process a NTSC signal, it is preferable that the tuner have a high-level of image rejection. However, less image rejection is acceptable for non-NTSC signals depending on the specific application and the corresponding display requirements.
To achieve a high level of image rejection, traditional TV tuners utilize a dual-conversion architecture having two mixers and at least one surface acoustic wave (SAW) filter. The first mixer up-converts the received RF signal to a first IF frequency (e.g. 1220 MHZ) that is fixed above the RF signal band of the incoming TV signal, using a variable local oscillator (LO) signal. A SAW filter, centered at the first IF, selects the channel of interest and provides the image rejection to prevent signal interference. The second mixer then down-converts the first IF to a lower frequency second IF, using a second fixed frequency LO signal. The second IF output is at baseband for a NTSC compatible signal. Alternatively, the second IF is at 36 or 44 MHZ for a cable system applications, such as for further processing in a set top box or a cable modem. Channel selection is realized by adjusting the first LO signal so that the desired channel is up-converted into the passband of the SAW filter, and is then down-converted to baseband by the second mixer and its second LO.
When the tuner is configured to output the desired channel at second IF (e.g. 36 MHz or 44 MHz for cable), an image channel or frequency is also down-converted to the second IF that interferes with the desired channel. For standard TV channels, the second IF is located at the IF1-2·IF2. For example, for an IF1=1220 MHz and a IF2 at 44 MHz, then the image channel will be located at 1132 MHz. Conventional SAW filters attenuate the image channel by approximately 45-50 dB. However, NTSC standards require from 60-65 dBc of image rejection to avoid degradation, whereas cable modems can live with 50-55 dBc of image rejection to avoid degradation.
One conventional technique to reduce the amplitude of the image channel, and the resulting interference, is to use an image reject mixer for the second stage mixer. However, image reject mixers utilize two component mixers in an IQ configuration and therefore require increased local oscillator drive and twice the power consumption when compared to single mixer configurations.
A second technique is to implement a narrowband notch filter at the input or output of SAW filter that is intended to reject only the image frequency (e.g. 1132 MHz). Single frequency notch filters, however, are extremely difficult to build and their accuracy can vary with the input or output impedance of the SAW filter.
What is needed is a method or apparatus for improving image rejection in a dual conversion receiver that does not require image reject mixers or conventional notch filters.